This invention relates to a system for treating particulate-laden boiler flue gas with a conditioning agent to improve the removal of particulate matter by electrostatic means, and, more particularly, relates to a conditioning agent demand control apparatus used in controlling a flow of conditioning agent.
The increasing demand for electrical power has forced electrical utilities to burn increasing quantities of fossil fuels such as coal. However, electric utilities face increasing environmental standards that are imposed upon their operation. To reduce air pollutants, electrical utilities generally use a flue gas treatment system intended to remove the majority of the particulate matter in the gas effluent. A flue gas treatment system generally comprises an electrostatic means, such as an electrostatic precipitator, and a conditioning agent source for introducing a conditioning agent into the flue gas stream.
An electrostatic precipitator generally comprises at least one set of electrodes disposed within the flue gas stream path and positioned in the flue conduit. A transformer and rectifier apply DC (direct current) high voltage power to charging electrodes to produce a high voltage field between a charging electrode and a collecting electrode. Entrained particles travelling in the flue gas stream are charged and attracted to the collecting electrode in the electrostatic precipitator.
The efficiency of an electrostatic precipitator in removing particulate matter from the boiler flue gas is dependent, in part, upon the electrical resistivity of the entrained particulate matter in the boiler flue gas. The entrained particulate matter expelled from a boiler fired with low sulfur coal, i.e., coal having less than 1 percent sulfur, has been found to have a resistivity of approximately 10.sup.13 ohms/cm. It has been determined that efficient removal of particulate matter by electrostatic precipitation occurs when the particulate matter resistivity is approximately 10.sup.8 ohms/cm. Therefore, to obtain effective use of an electrostatic precipitator, the resistivity of the entrained particulate matter from low sulfur content coals must be reduced.
Electrical utilities have long used conditioning agents introduced into the flue gas stream prior to entry into the electrostatic precipitator to reduce the resistivity of the entrained particles. Various chemicals, such as water, anhydrous ammonia, and various ammonia-bearing solutions, sulfuric acid, sulfur trioxide and phosphoric acid, have been used as conditioning agents. Controlling the flow of the conditioning agent has been approached in a variety of ways.
U.S. Pat. No. 2,864,456 discloses an automatic control for electrostatic precipitators which varies both the electrostatic precipitator voltage and the supply of a conditioning agent, such as water, for conditioning particles to be removed by the electrostatic precipitator and to maintain an optimum sparking rate for efficient particle removal.
U.S. Pat. No. 3,523,407 discloses a method of improving the electrostatic precipitation of particles from a flue gas by adding preselected amounts of ammonia and water to the flue gas.
U.S. Pat. No. 3,665,676 discloses a system to condition the particles of boiler flue gas by the use of a salt solution such as a solution of ammonium sulfate or ammonium bisulfate. The salt solution is injected into the flue gas prior to entering the electrostatic precipitator and the system includes a metering means for controlling the amount of conditioner injected into the flue gas. U.S. Pat. No. 3,665,676 indicates that, if desired, conventional automatic controls can be provided to open the metering means when the flue gas reaches the desired operating temperature or to close it should the temperature fall below operating temperature. In addition, automatic controls can also be made to open the metering means to provide the amount of conditioner needed in proportion to the volume of gas to be conditioned.
U.S. Pat. No. 3,689,213 discloses a process for treating flue gas in which gaseous sulfur trioxide is generated in the immediate vicinity of the point of use as required by the quantity of fossil fuel being burned per unit time and is then introduced into the flue gas at a predetermined rate to facilitate fly ash removal by an electrostatic precipitator.
U.S. Pat. No. 3,772,178 discloses a system for the production of sulfur trioxide for flue gas conditioning including means to deliver a source of sulfur such as sulfuric acid to a vaporizer in proportion to the amount of flue gas from the boiler measured in terms of the electrical output generated at a particular time. As the production of flue gas changes in the boiler system, the proper ratio of acid to flue gas is automatically maintained by a control responsive to a signal coming from a boiler capacity index gauge to control the volume of sulfur trioxide being produced.
U.S. Pat. No. 3,993,429 discloses a system in which the temperature of operation of the catalytic converter is controlled by providing a second flow of air to be mixed with the output of the sulfur burner, detecting the temperature of the mixture of the second flow of air and the gases leaving the sulfur burner and varying the temperature of the air in the second flow of air to maintain a desired operating temperature for the catalytic converter. U.S. Pat. No. 3,993,429 further discloses that SO.sub.3 flue gas conditioning systems can operate by sensing the rate of coal combustion and varying the rate of flow of sulfur into a sulfur burner in response to the rate of coal combustion.
U.S. Pat. No. 4,770,674 discloses a system for conditioning flue gas for an electrostatic precipitator, including equipment for converting sulfur into sulfur trioxide. The disclosed systems of U.S. Pat. No. 4,770,674 include a sulfur burner to produce oxidized sulfur, a catalytic converter to convert the oxidized sulfur to sulfur trioxide, and means to control sulfur and air inputs to the sulfur burner. Various inputs to the control means are disclosed, including the outlet temperature of the catalytic converter and such operating parameters of the exhaust stage of the system as the output temperature of the exhaust gas from the precipitator, the flow rate of the exhaust gas, the power delivered to or the speed of, an induced draft fan, if any, the opacity of the exhaust gas within the stack, and the power dissipated by the precipitator.
U.S. Pat. No. 4,779,207 discloses a system for preconditioning flue gas for electrostatic precipitation. The system of U.S. Pat. No. 4,779,207 includes a source of an SO.sub.3 conditioning agent, a means for controllably adding the conditioning agent to the flue gas, a means for detecting the input power level of the electrostatic precipitators and control means for monitoring the input power level and controlling the amount of conditioning agent added to the gas to substantially maintain input power to the electrostatic precipitator to predetermined levels.
A controller commercially available from Castlet (Electronic Engineers) Ltd., of 14 Crofton Drive, Lincoln, England, can control an electrostatic precipitator by detecting the presence of deleterious back ionization and intermittently applying voltage to the charging electrodes of the precipitator to minimize back ionization. The Castlet controller detects back ionization by interrupting the applied charging voltage at its peak value and comparing, after a preset time, the actual charging electrode voltage with a programmed charging electrode voltage to identify excess charging electrode decay rate indicative of back ionization. The Castlet controller uses the difference in actual and programmed charging electrode voltage to determine a rate of application of voltage to the charging electrodes in an effort to optimize precipitator operation in the presence of conditions of back ionization.
U.S. Pat. No. 5,032,154 discloses, among other things, a system which provides direct, automatic control of the opacity of the effluent of a coal-fired boiler to maintain minimal opacity of the flue gas effluent passing from the boiler into the atmosphere. Systems of U.S. Pat. No. 5,032,154 provide a controlled flow of an agent, such as sulfur trioxide, to the boiler flue gas to condition its particulate matter for removal by electrostatic means, monitor precipitator power, and the opacity of the boiler flue gas after it leaves the electrostatic particle-removal means, and vary the controlled flow of conditioning agent to hunt and operate at conditioning agent flow rates determined from flue gas opacity alone or combined with precipitator power.
Other conditioning systems are shown, for example, in U.S. Pat. Nos. 3,686,825; 3,993,429; 4,042,348; 4,466,815; 4,533,364; and 4,624,685.